BIOSIGNAL ACQUISITION DEVICE

A Novel Topology for Wearable Signal Acquisition Devices

Luca Maggi, Luca Piccini, Sergio Parini, Giuseppe Andreoni

Dipartimento di Bioingegneria of Politecnico di Milano - Milan, Italy

Guido Panfili

SXT – Sistemi per telemedicina srl. – Lecco, Italy

Keywords: Wearable device, Brain Computer Interface, Amplifier, Signal Conditioning, Offset Recovery, Low Voltage

Amplifier, Band Pass Filtering, ECG amplifier, EEG amplifier.

Abstract: The here presented work illustrates a novel circuit topology for the conditioning of biomedical signals. The

system is composed of an amplification chain and relies on a double feedback path which assures the

stability of the system, regardless of the amplification block gain and the order of the low-pass filter

settings. During the normal operation, the offset recovery circuit has a linear transfer function, when it

detects a saturation of the amplifier, it automatically switches to the fast recovery mode and restores the

baseline in few milliseconds. The proposed configuration has been developed in order to make wearable

biosignal acquisition devices more robust, simpler and smaller. Thanks to the used AC coupling method,

very low high-pass cut-off frequencies, can be achieved even using small valued passive components with

advantages in terms of circuit bulkiness. The noise rejection filter between the pre-amplification and the

amplification stages eliminates the out-of-band noise before the amplification reducing the possibility of

having clipping noise and minimizing the dynamic power consumption. The presented topology is currently

used in a prototypal EEG acquisition device in a Brain Computer Interface (BCI) system, and in a

commercial polygraph which will be soon certificated for clinical use.

1 INTRODUCTION

Wearable systems ought to be totally unobtrusive

devices that allow physicians to overcome the

limitations of standard ambulatory technology,

aiming at providing a response to the need for

monitoring individuals over weeks or even months

without or limiting their usual behaviour. Such a

systems typically rely on wireless, miniature sensors

embedded in patches, bandages, or in items that can

be worn, such as a ring or a shirt. They take

advantage of hand-held units to temporarily store

physiological data, which can be uploaded

periodically to a database server through a wireless

LAN or different gateways that allow Internet

connection. The data sets recorded using these

systems are then processed to detect events able to

indicate a possible worsening of the patient’s clinical

situation or providing information explored to assess

the impact of clinical interventions (Park, 2003).

Wearable devices are usually battery powered:

low voltage supply and low power consumption are

mandatory features for this kind of devices, in order

to provide a good battery life to dimension ratio. In

the last 10 years many garments with embedded

sensors have been developed: the intrinsic

characteristics of such electrodes and the possible

instability of the contact make the design of

wearable acquisition devices more difficult

(Webster, 1991). The main aspects we have to take

into account in the design of a wearable surface

biopotential amplifier (e.g. Electrocardiogram-ECG,

Electroencephalogram–EEG and Electromiogram-

EMG) are:

• Dynamic reserve;

• Max offset rejection;

• Fast recovery from artefacts.

Although the operational amplifiers production

technology has developed several low power and

low cost devices, the development of dedicated

397

Maggi L., Piccini L., Parini S., Andreoni G. and Panﬁli G. (2008).

BIOSIGNAL ACQUISITION DEVICE - A Novel Topology for Wearable Signal Acquisition Devices.

In Proceedings of the First International Conference on Bio-inspired Systems and Signal Processing, pages 397-402

DOI: 10.5220/0001059703970402

 SciTePress

topologies is still necessary in order to maximize the

overall circuit performance.

2 METHODS

2.1 Background

Figure 1 shows a typical, state of the art biosignal

detection circuit which is composed of a set of

independent stages connected in a chain. At the

beginning there is a pre-filtering stage, the pre-

amplification stage which is followed by the offset

rejection circuit and by an amplification and filtering

circuit.

This kind of solution is simple and effective when

the wide power supply range provides a high

dynamic reserve (avoiding clipping problems) and

when the mechanical specifications allow the use of

high capacity capacitor or the specific application

doesn’t require very low frequency high pass filter.

It is worth to underline that the maximum tolerable

offset is given by the following equation:

preamp

MAX

Off

)(

supsup

−+

−

≅

(1)

Where V

sup

are the value of the supply rails and

preamp

is the gain of the preamplification stage.

It is worth noting that in case of a change in the

input signal that causes the amplifier saturation, the

output of the system will remain latched for a time

which depends on the signal amplitude; it is possible

to overcome this limitation by increasing the system

complexity and inserting a baseline reset circuit

which is activated by the saturation of the system

itself.

2.2 General Description

The proposed system is composed of a differential

pre-amplification stage P(s): usually realized using

an Instrumentation Amplifier (INA). The F(s) block

is a unity gain inverting filter (low-pass or low-pass

plus notch filter) of any order. A(s) is an

amplification stage, while I(s) is an offset

compensation network. In the proposed version it is

a non-linear circuit which acts as an attenuated

inverting integrator when the Vin is inside the linear

region and as an amplified inverting integrator when

the signal is over threshold whose behaviour can be

expressed as follow:

sup sup

if V Th V V Th

asRC

kotherwise

sRC

−+

⎧

−

+<< −

⎪

⎨

⎪

−

⎪

⎩

(2)

where Th is threshold value which identifies a

saturated state, ‘a’ is an attenuation factor and ‘k’

the amplification factor.

The small signal transfer function and the G

Loop

of the system are represented by the following

equation:

)](1)[()(

]1)()[()(1

)()(

sFsIsAG

sFsAsI

sAsF

sPsTF

LOOP

−=

−−

⋅=

(3)

Figure 1: The amplification chain proposed by the OpenEEG project.

BIOSIGNALS 2008 - International Conference on Bio-inspired Systems and Signal Processing

398

Figure 2: Block structure of the system.

considering that I(s) and F(s) are inverting the

equation can be expressed as follows:

]1)([)()(

]1)([)()(1

)()(

+⋅⋅−=

+⋅⋅−

⋅=

sFsIsAG

sFsAsI

sAsF

sPsTF

LOOP

(4)

The transfer function is a band-pass amplifier with a

single pole high-pass and a low-pass whose shape

depends on F(s). Figure 3 shows the bode diagram

of a system with the following characteristics:

• F(s): 2

order low pass at 75Hz;

• A(s): amplifier gain 100 V/V;

• P(s): pre-amplifier gain 5V/V;

• I(s): integrator 1/100 * 1/s.

Figure 3: Frequency response of a sistem with 500V/V

gain and a 2nd order low pass 75Hz filter.

2.3 Offset Compensation Issues

On the basis of the final output, the offset

compensation value is fed both directly to the

preamplifier P(s) reference pin, and by modifying

polarization of the amplifier A(s). The proposed

structure introduces a systemic offset compensation

method which ensures that, thanks to the double

feedback path, even when the pre-amplification

output is close to the power supply rail, the

following stages work inside the linear region this

property doubles the maximum tolerable offset:

pream

MAX

Off

)(

supsup

−+

−

≅

(5)

Thanks to this improvement it is possible to increase

the gain of the pre-amplification stage taking major

advantages of the qualities of the INA in terms of

CMRR a noise figure.

As proposed in our previous work, the AC-

coupling of the amplifier using a feedback integrator

allows the tuning of the high-pass pole frequency

just by varying the open loop gain of the system

(Maggi, 2004). When setting the parameters for

biosignals acquisition, it is useful to insert an

attenuation factor in the I(s) block in order to

compensate the amplifier gain: keeping the G

loop

below the unity gain the high pass pole is moved to

the lower frequencies.

The I(s) automatically identifies a saturation of

the amplifier using a threshold method: if the value

is outside a predefined interval, the attenuated

integrator is switched into an amplified integrator

that quickly brings the system output inside the

linear interval.

The k value defines the delay of the offset

recovery of the system: for example we can have a

0.05Hz high pass pole during the linear phase and

switch it to a 100Hz one during the offset recovery

phase, achieving a baseline recovery in about 10ms.

2.4 Stability of the Loop

During the normal operation the G

loop

is usually kept

low using the attenuation net of I(s) in order to

achieve the desired high-pass frequency; when the

BIOSIGNAL ACQUISITION DEVICE - A Novel Topology for Wearable Signal Acquisition Devices

399

saturation occurs the I(s) is switched to a high gain

configuration: in this section the stability of this

configuration will be discussed both by considering

the Bode Stability Criterion and the root-locus

method.

2.4.1 Bode Stability Criterion

Provided that A(s) have a sufficient bandwidth to be

considered like an ideal amplifier and that F(s) and

I(s) are stable, the poles in F(s) and I(s) are the

possible instability causes of the system.

Considering the open-loop transfer function, the I(s)

provides a single pole at low frequencies, while the

F(s) put a variable number of poles at the higher

bandwidth limit. Thanks to the second feedback path

the poles of F(s) are compensated by the same

number of zeroes. The figure 4 shows the Bode

diagram of the original F(s) and the compensated

one.

The newly created zeros must be very close to the

F(s) poles in order to allow a difference between the

DC gain and the high frequency one of just 6dB. The

nearness of poles and zeroes makes the phase plot

very flat: for any complexity of the filter the plot is

between +90 and +270 degrees.

Figure 4: Bode plot of the compensate filter against the

original one.

Figure 5 shows the phase diagram of the resulting

loop

, including the pole introduced by the

integration process, it is possible to notice that it

never cross the instability region.

Figure 5: Phase diagram of the resulting Gloop.

2.4.2 Root Locus Study

The root locus (figure 6 and 7) show that the all the

resulting closed loop poles are in the left semi-plane

even with a 9th order low pass filter. For the higher

open loop gains the phase margin can be less than 45

degrees, but during the nonlinear phase the

overshoot can make the settling faster.

Figure 6: Root locus of a 4th order system.

Figure 7: Root a 9th order filter system.

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400

3 RESULTS

The configuration has been adopted both in a

commercial wearable polygraph, in order to acquire

the ECG signal and on a EEG acquisition prototype

devoted to Brain Computer Interface applications.

Figure 8 shows the proposed implementation for the

EEG acquisition device. The system is 3,3V single

supply powered using a li-ion battery and a low-

dropout linear voltage regulator. The

preamplification stage has a gain of 100V/V and

P(s) is realized using a INA118 (Texas Instruments).

The other four operational amplifiers are contained

in a single integrated circuit (TLC2254, Texas

Instruments).

The F(s) is an inverting double pole low pass

filter, and A(s) is an amplification chain. The I(s) is

composed by and attenuation network (R30 and

R31), an inverting integrator (IC2B, C1 and R12)

and the nonlinear activation network (K2, H2, R22,

R23, R25, R27, R26).

The R27 and R26 network are used in order to

set the intervention threshold of the offset recovery

circuit: when the V

of K2 and H2 are kept below

0,7 Volt the transistor are turned off. The Th

parameter is defined also follows:

2627

7,0

VTh

⋅=

(6)

K2 is switched on when the amplifier output voltage

reaches the upper saturation limit, while H2 is

switched on in case of lower saturation. When one

of the transistor is turned on, it injects a current into

the inverting integrator causing the fast offset

recovery. R22, R23, R25 are necessary in order to

limit the transistor current and avoid instabilities

related to 2

order effects of the components. The

final amplification stage is optional an provides a

last anti aliasing filtering.

The system circuit has been successfully used in a

brain computer interface application (Piccini, 2005

and Maggi, 2006) and has an offset recovery time of

less than 10ms.

4 DISCUSSION

The proposed architecture is a smart and cost

effective solution to the problems related to the

acquisition of biosignal in difficult acquisition

situations based on an analog design; thanks to the

evolution of modern digital devices, it is possible to

adopt other method in order to achieve similar

results.

The strength of the proposed topology is that a

simple local solution doesn’t require a full systemic

redesign and support development of modular

multiparametric wearable devices.

The discussion doesn’t take into account second

order effects related the components physical

limitations: even if the architecture is robust and the

frequency range of biosignals is reduced, the design

of an amplifier based on the proposed topology

should be approached with care.

Figure 8: Schematic of the EEG amplification circuit.

BIOSIGNAL ACQUISITION DEVICE - A Novel Topology for Wearable Signal Acquisition Devices

401

5 CONCLUSIONS

The analysis proposed in this paper shows an

interesting approach for providing a cost effective

solution for AC coupled, low power amplifiers.

Although born in a biomedical research laboratory,

it faces problems related to a wide range of different

applications. Also for this reason, this generic

topology has been patented in Italy, and successfully

revised by the European Patent Office for the PCT

extension.

ACKNOWLEDGEMENTS

This work has been partially supported by ST

Microelectronics and IIT Istituto Italiano di

Tecnologia.

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